This invention will be described as applied to the aluminum wire. The term "aluminum" is used herein to designate pure aluminum, EC grade, other major grades of aluminum containing normal amounts of impurities, and alloys of aluminum in which aluminum is the major ingredient. It will be understood that the invention can be used for elongated aluminum stock other then wire, for example, aluminum strip.
Aluminum wire clad with copper is the equivalent of solid copper with respect to maintaining a low electrical contact resistance and is acceptable in most applications. A low-cost commercial method for applying a copper cladding can provide a cost savings because aluminum conductors cost less per unit of current carrying capacity than solid copper conductors. Electroplating is a commercial method for applying copper coating on steel wire and should be equally successful for aluminum. However, other investigators have been unable to obtain quality copper electroplates on moving aluminum wire. Either the adherence or ductility of the copper deposit was poor, or the processing time was too long.
Most procedures for plating on aluminum were developed for conventional tank plating where there is relatively little agitation or flow of solution over the aluminum surfaces. Plating on moving wire imposes different conditions because there is relative motion between the aluminum and the processing solutions. For wire plating this motion can exceed 100 ft per minute.
This invention provides a method and apparatus for applying a plating of adherent ductile copper on aluminum wire which is traveling continuously through the plating apparatus. The combination of successive steps of this invention obtains the desired plating rapidly so that the process can be carried out with the wire traveling at high speed without making the apparatus excessive in length. After the preliminary cleaning and passivating, the invention passes the wire through a zincating step followed by a copper strike plating. The amount of copper applied in the strike plating has been found to have critical limits which are important to the success of the subsequent steps of the process. The thickness of the copper plating is built up by subsequent plating in a copper fluoborate bath.
The certain chemical compositions of the baths used for different steps of the process have been found much more advantageous than others.
Other objects, features and advantages of the invention will appear or be pointed out as the description proceeds.